Growth of crystalline rare earth iron garnets and orthoferrites by vapor transport

ABSTRACT

A process for the production of crystals of the yttrium and rare earth iron garnets and orthoferrites by vapor transport using HC1 or Cl2 has been developed. It has been found that the addition of s sufficient amount of FeC13 to the reactants suppresses the decomposition of the deposited material and allows the production of crystals of excellent quality. The process is adaptable to sealed capsule or open tube operation.

United States Patent Inventor Friedel H. P. Wehmeier Murray Hill, NJ.

Appl. No. 849,351

Filed Aug. 12, 1969 Patented Oct. 26, 1971 Assignee Bell Te!ephoneLaboratories, Incorporated Murray Hill, NJ.

GROWTH OF CRYSTALLINE RARE EARTH IRON GARNETS AND ORTHOFERRITES BY VAPORTRANSPORT 7 Claims, 2 Drawing Figs.

U.S. Cl. 23/21, 23/51, 117/106, 117/235 Int. Cl C22b 59/00 Field ofSearch 23/21, 51; 117/106, 169, 235

References Cited UNITED STATES PATENTS 4/1964 Gambino 3,178,313 4/1965Moest 117/106 X 3,421,933 l/1969 Pulliam 117/106 X 3,429,740 2/1969 Mee117/106 OTHER REFERENCES Mee et al., Applied Physics Letters," Vol. 10,May, 1967, pp. 289-291.

Primary ExaminerHerbert T. Carter Alzorneys-R. J. Guenther and Edwin B.Cave GROWTH OF CRYSTALLINE RARE EARTH IRON GARNETS AND ORTHOFERRITES BYVAPOR TRANSPORT BACKGROUND OF THE INVENTION l. Field of the InventionYttrium and rare earth iron garnets and orthoferrites can be depositedby vapor transport for possible application in microwave and memorydevices. These materials can be deposited as bulk crystals or asepitaxial layers on suitable substrates.

2. Description of the Prior Art The yttrium and rare earth iron garnetsand orthoferrites being magnetic insulators have found application in alarge variety of high-frequency devices such as microwave nonreciprocaldevices, acoustical devices (which may make use of theirmagnetostrictive properties), magneto-optic devices, and as storageelements in magnetic memories. The production of crystalline bodies ofthese materials has been accomplished in a number of ways including thegrowth from a flux and the sintering of powders. A technique which hasrecently seen active development is the production of these crystallinematerials by vapor transport.

Vapor transport using only an inert gas carrier is impractical becausethese materials are unstable at the temperatures which would be requiredfor this process. A more promising process is vapor transport using achemical reacting gas. In this process, the carrier gas reacts with thestarting materials forming volatile species which are subsequentlydeposited in a region of the system which is maintained at a lowertemperature. In various procedures, the starting materials can bechemically identical to the required product or they can be constituentmaterials which are deposited as the final product. Chemical reaction ofthe reactants can take place in a sealed system in which theconstituents are usually supplied in approximately stoichiometricquantities or in an open system in which there is a gas flow maintainedfrom the region of the starting materials to the region in whichdeposition takes place. In this latter case the vapor flows may beregulated to maintain the proper partial pressures'in the region ofdeposition.

Even though these processes using chemically reacting gases take placeat temperatures lower than would be necessary for inert gas vaportransport, they are still subject to chemical instabilities and it hasbeen difficult to realize satisfactory crystal growth. For example, whenattempting to grow yttrium iron garnet using I-ICl as a chemicallyreacting transport gas and using yttrium iron garnet as a startingmaterial, it is found that only Fe,0, is transported and Y,O is leftbehind. The most successful attempts appearing in the literature to date(Mee et al. Appl Phys Lett, Vol (1967) 289) involve the use of (C1 andFeCl, as the source materials and a combination of l-le,HCl, I-l,0 andO, as the carrier gases.

SUMMARY OF THE INVENTION It has been found that inclusion of a suitablequantity of l-eCl; in addition to the otherwise accepted sourcereactants stabilizes the transport reaction and the product crystalline'material. This allows the deposition of crystalline bodies without thepresence of spurious reaction products. The overpressure of FeCl: allowsa simple one-carrier gas system in which either BC] or Cl, alone canproduce successful crystal deposition. This invention is applicable overa wide range of conditions. The source reactants can be composed ofsimply the product material and a carrier gas or they can be acombination of a carrier gas with the yttrium or rare earth chloride,FeCl, and O, or H,O or the carrier gas with the yttrium or rare earthoxide, FeCl; and O, or H O. If the carrier gas is C], then theadditional gas will be 0,. If carrier gas is HCl then H, 0 will beneeded. While not necessary, various gases which do not take part in thechemical reactance can be included in order to suitably adjust thegaseous partial pressures.

The invention is adaptable to sealed systems or open ended systems andthe crystalline material can be deposited as a bulk crystal or asepitaxial layers on substrates present in the region of deposition. Theutility of this system is limited at the upper end of the pressure scaleby the need to maintain the partial pressure of the rare earth chlorideless than its partial pressure over its own liquid and at the lower endby the need to maintain practical rates of material transport. Suchtransport becomes impractically slow if the partial pressure of thecarrier gas is less than 10" atmosphere.

When working with a sealed system it is convenient to load the systemwith carrier gas at 1 atmosphere. Thus, at usual temperatures ofoperation, the internal partial pressure of the carrier gas is of theorder of 4 atmospheres. In an open system, it is convenient to operateat a total pressure of l atmosphere in which case inert gases might beused to adjust the various partial pressures to the appropriate values.The sealed capsule operations take place with the source materialstypically, between 1,000 C. and l,600 C. and a temperature differencebetween the source end and the deposition end of the system greater than10 C. Temperature differences of C. have been used. These still yieldconveniently controllable deposition and do not represent an upperlimit. Temperature differences less than 5 C. result in impracticallyslow deposition rates. In open tube operation, where the carrier gas iscaused to flow from the source end of the vessel to the depositionregion, temperature differences as described above may not be required.In some situations, the temperature at the source end of the vessel maybe lower than the temperature in the deposition region.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a plane view in section of asealed vapor transport capsule showing bulk crystal growth and epitaxialgrowth on a substrate; and

FIG. 2 is a plane view in section of an open tube vapor transport vesselshowing bulk crystal growth and epitaxial growth on a substrate.

DETAILED DESCRIPTION OF THE INVENTION Chemistry of the Systems UnderConsideration The yttrium and rare earth iron garnets and orthoferritesare ternary compounds composed of rare earth elements, iron and oxygen.The garnets have the general formula R mo The orthoferrites have thegeneral formula R Fe 0 where R is yttrium or a rare earth elementselected from the list Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er Tm, Yband Lu.'According to the invention disclosed here, crystalline bodies ofthese materials are produced by vapor transport using either HCI or Cl,as the chemically reacting carrier gas and including in the depositionchamber an overpressure of FeCl;.

In the garnet system, the reactants are transported according to thefollowing chemical equation which will be hereinafter referred to as thetransport reaction;

if HCl is used as the carrier-gas. If Cl, is used as the carrier-gasthen 0 appears in the right-hand side of the transport reaction equationin place of H 0 with appropriate balancing. All of the products on theright-hand side are gases at the temperature of transport.

In order to accomplish successful transport another reaction must alsobe considered. This will be referred to as the decomposition reaction.This reaction follows the chemical equation;

Here, at the transport-temperature R 0, is a solid and in many caseswill be deposited in the region of the source. The transported vapors,then, lose their stoichiometry and unwanted compounds are deposited atthe deposition end producing imperfect crystals. If successful transportis to be accomplished, conditions must be maintained so that the freeenergy of this reaction is positive. If this condition is met, thedecomposition reaction will proceed towards the left making the ternaryphase stable both in the source region and in the deposition reiiitheorthoferrite system, the transport reaction is;

RFeO,+HCl=RCl,bzFeCl,-+-3H,0. 3

The corresponding decomposition reaction is;

RFeO,+HCl=0.5 R,O,+FeCl;,+l .5H,O (4) (If Cl, is used as a carrier gas,O will be produced in place of ",0 as above.)

Thermodynamics of the Systems Under Consideration The free energy (AG)of any of the above reactions can be calculated in tenns of tabulateddata and the partial pressures of the gaseous reactants according to theformula;

AG=AG+RTInK (5) In equation (5) AG" is the free energy of the reactionat standard conditions (gaseous constituents at 1 atmosphere pressure)and can be calculated at any temperature from tabulated data (0.Kubachewski and E. Ll. Evans, Metallurgical Thermochemistry 4th Ed.,Pergamon Press) and ordinary thermodynamic relations. R is the universalgas constant, T is the absolute temperature and K is the constantappearing in the law of mass action. For instance, for the decompositionreaction in the garnet system (6) where p( FeCl,), for instance, is the5th power of the partial pressure of FeCl,. Noting that the exponentsappearing on the right of equation (6) correspond to the stoichiometriccoefficients of the species in equation (2), a corresponding relationfor any of the chemical reactions contemplated can be written. It hasbeen found that for the systems under consideration AG can be madepositive by including, in addition to the source reactants an amount ofFeCl; in order to produce an overpressure of FeCl, in the vessel.

The following is a suggested procedure for the determination of thisadditional amount of FeCl;,. Assuming a particular HCl partial pressure,the partial pressure of FeCl;, which represents the equilibriumcondition for the decomposition reaction (equation (2) must be found.This is found by setting AG in equation (5) equal to zero and notingthat at equilibrium the partial pressure of 11,0 equals 1.5 times thepartial pressure of FeCl,. This relation in addition to tabulated dataand the assumed pressure of HCl permits the calculation of theequilibrium partial pressure of FeCl;,. in order to make AG positive, anadditional amount of FeCl, must be incorporated in the system so as toincrease the partial pressure of FeCl, at least of the order of 1percent above equilibrium value Depending upon the accuracy of thetabulated data used in this calculation and the temperature gradient inthe vessel, an additional margin may be desired. Margins as great as 100percent have been used, but they do not represent an upper limit ofutility. It the temperatures difference between the source region andthe deposition region of the apparatus is large, the calculation shouldbe done at both temperatures in order to make sure AG is still positiveand the deposited material is stable.

Given a set of conditions appropriate to one pressure level, thereaction conditions appropriate to any other pressure level can bederived directly from the consideration of equation (6). Assuming, forinstance, that it is desired to work at onetenth of the original HClpressure one need only adjust the partial pressureof FeCl, and l-l,O soas to keep AG of the reaction of equation (2),positive.

This procedure would be useful if, for instance, it is desired to takethe results of a sealed capsule experiment and apply them to an opentube apparatus.

The knowledgeable practitioner can directly extend this procedure to theorthoferrite system and to the use of Cl, as'a carrier gas by suitablytreating the partial pressures of the various species in thecorresponding balanced decomposition equation.

Starting Materials The various possible sets of source reactants can beobtained from consideration of the transport and the decompositionreactions. In addition to the carrier gas and the additional amount ofFeCl; calculated as above, the starting materials can be (1) chemicallyidentical to the desired product material or its binary constituentoxides in the form of crystal pieces, polycrystalline bodies or sinteredmaterials (perhaps in crushed or powder form in order to increase thesurface area for chemical reaction), (2) the reactants on the right sideof the transport reaction; (3) the reactants on the right side of thedecomposition reaction; or (4) the reactants on the right side of areaction similar to the decomposition reaction but including rep, andR0,. If source materials (2), (3) or (4) are used, it is usuallydesirable to maintain the required partial pressures of the gaseousspecies by including appropriate amounts of the source materials 12 in asealed system 11 (see FIG. 1) or by regulating the flow of the variousmaterials 22, 221 in an open system 21 (see FIG. 2). This flow can beregulated by such means as variation of orifice size or selectiveheating of the various constituents. in many applications, it isdesirable to alter such properties of the pure ternary material (thegarnet or the orthoferrite) as the saturation magnetization. This can bedone by the partial substitution of a nonmagnetic species for a portionof one of the magnetic species or by the partial substitution of aspecies with a different magnetic moment. Such species as Al and Ga havebeen used for this purpose. In such a case the corresponding freeenergies must be considered and it may prove necessary to include anoverpressure of the corresponding chloride (e.g. AlCl or GaCl in orderto stabilize the reaction.

Example As a practical example of the application of this invention, thevapor transport of yttrium iron garnet (YIG) has been per formed. Thismaterial is particularly important in high frequency electromagnetictechnology today. The decomposition was performed in a sealed capsuleusing HCl as a carrier gas and polycrystalline YIG as a source material.As a matter of convenience the capsule was filled with HCl atatmospheric pressure and sealed. This produced a pressure ofapproximately 4 atmospheres at the reaction temperature. Using 4atmospheres as a starting partial pressure of HCl, the equilibrium ofFeCl, was calculated as discussed in the procedure above asapproximately 2.5 atmospheres. The capsule was charged with 1 atmosphereof HCl, polycrystalline Y;,Fe,0,,, and 8 milligrams of FeCl; per cubiccentimeter of capsule volume. The source end of the capsule wasmaintained at 1,140 C. and the deposition need of the capsule wasmaintained C. lower. These conditions produced a total par-' tialpressure of FeCl;, approximately equal to 5 atmospheres. At the end ofone week 1,200 milligrams of bulk single crystals 13 of Y,re,o,, wereproduced at random nucleation sites in the deposition region. If asuitable substrate 14 had been supplied, the 1,200 milligrams ofmaterial would have produced epitaxial film 15 of 10 micron thicknessesover a 200 square centimeter area.

What is claimed is:

1. A process for the growth of a crystalline body of a materi alcontaining at least, one compound selected from the group of ternarycompounds consisting of the yttrium and rare earth iron orthoferritesand the yttrium and rare earth iron garnets in a reacting vessel,maintained between 900 C. and l,600

C., by vapor transport from a source region, containing stoichiometricamounts of (1) RCl,, FeCl, and l-l,0 or 0,, or (2) R,O,, FeCl, and l-l,0or 0,, or (3) Fe,0,, RC1, and H or 0,, or (4) R,Fe,0,, or RFeO,, or (5)12,0, and Fe o as source reactants containing HCl when H 0 is a sourcereactant, Cl, when 0, is a source reactant or HCl or Cl, when neither",0 or O, is a source reactant.

2. A process of claim 1 in which the partial pressure of the carrier gasis greater than atmospheres.

3. A process of claim 2 in which the said reacting vessel is sealed.

4. A process of claim 3 in which the temperature of the said l 1' i l iUNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION 3, 615, 168 DatedOctober 26, 1971 Patent No.

lnventor(s) Friedel H, P. Wehmeier It is certified that error appears inthe above-identified patent and that said Letters Patent are herebycorrected as shown below:

line 10, that portion reading "10 should read lO Column 3, line 9,equation (3) should be changed to read RFeO 6HC1 R C1 FeCl 31-1 0 Column3, line 12, equation t) should be changed to read as follows: RFeO 3HCl0.5 R 0 FeCl 1.5 H 0",

Column l, lines #8 and M9, the word 'decomposition" should bedeposition.

Column 5, line 5 ,in claim 1, after "reactants" insert where R is oneelement selected from the group consisting Column 2,

of Y, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, I'm, Yb and Lu, to adeposition region using at least one carrier Line 7, same column, afteris a source reactant insert characterized in that the said reactingvessel also contains FeCl as an initial ingredient in such quantity asto produce a positive value of the free energy, AG, of

the decomposition reaction embodied in the chemical equation containingthe said compound and the said carrier gas on the left side and theoxide of the said rare earth,

ferric chloride and the gas needed for balance on the right side whereAG AG RT n K, AG is the free energy at standard conditions, R is theUniversal Gas Constant,

T is the absolute temperature and K constant appearing in the Law ofMass Action. 1L? 4 Column 5, line 9, that portion "10 u should read 10'Column 5, line 9, after atmospheres insert in the reacting vessel-.

(SEAL) Attest:

ROBERT GOTTSCHALK Co issioner of Patents EDWARD M.FLETCHER,JR. AttestingOfficer USCOMM-DC 80376-P09

2. A process of claim 1 in which the partial pressure of the carrier gasis greater than 10 4 atmospheres.
 3. A process of claim 2 in which thesaid reacting vessel is sealed.
 4. A process of claim 3 in which thetemperature of the said deposition region is lower than the temperatureof the said source region by at least 5* C.
 5. A process of claim 4 inwhich the said yttrium rare earth iron garnet is yttrium iron garnet. 6.A process of claim 2 in which the said reacting vessel is open and inwhich the carrier gas is caused to flow therethrough in a direction fromthe said source region to the said deposition region.
 7. A process ofclaim 6 in which the said yttrium and rare earth iron garnet is yttriumiron garnet.